1. Field of the Invention
The present invention is in the field of radar receivers employing post detection processing techniques to determine target information. Such radar receivers are generally employed in the area of MTI radars, normally mounted on aircraft, and operate in either search or track modes.
2. State of the Prior Art
In the prior art, the problem of rejecting ground clutter received through the main beam without rejecting true targets and the problem of distinguishing near range point clutter received through the sidelobes from true targets received through the main beam have been persistent.
A medium PRF pulse doppler (PD) type radar is characterized by having an ambiguous range measurement, for each received pulse. By contrast, in a low PRF system, the time period between each pulse transmitted therein allows time for the transmitted pulse to be reflected from a within-range target and to be received by the receiver before the next pulse is transmitted. Therefore, in a low PRF system, the range is unambiguous since the time between the transmitted pulse and the corresponding received signal is known. However, in a medium PRF system, the transmitter transmits a pulse, and prior to the pulse having time to be returned by a within-range target, a subsequent pulse is transmitted. The inter-pulse period (time between each transmitted pulse) in the medium PRF system is shorter than that for the low PRF system. Therefore, pulses received by the radar receiver in a medium PRF system are ambiguous in range, since the return signal may occur in an inter-pulse period subsequent to the period following the corresponding transmitted pulse.
The medium PRF system is useful, however, since a doppler shift can be detected in each returned signal. Each transmitted pulse is coherent with respect to every other transmitted pulse, and the PRF frequency is high enough to monitor high or low speed doppler shifts in the returned signals, allowing valuable information to be derived. The presence of doppler shift in the return signals provides an additional basis for distinguishing the return signal and determining if it is from a true target, main beam clutter, sidelobe clutter or ground moving targets.
In a typical prior art radar receiver as is shown in FIG. 1A, two separate receiver channels 10 and 20 are used in order to distinguish the true target return from main beam clutter and sidelobe clutter in a medium PRF system. A main channel coherent receiver 7 is connected through a duplexer 5 to the main antenna 1. The main antenna 1 is an antenna characterized by having a high gain directional main beam reception response and a low gain sidelobe reception response, as is shown in FIG. 1B. A separate guard receiver channel 20 includes a coherent receiver 8 connected to a guard antenna 2. The guard antenna 2 is separate from the main antenna and, as shown in FIG. 1B, has a characteristic broad beam response with a gain which is higher than any sidelobe of the main antenna and a gain which is lower than the main beam of the main antenna. The guard channel 20 will necessarily receive the same information as does the main channel 10. However, due to the construction of the respective main antenna and guard antenna, signals received in the main beam of the main antenna 1 are received with a higher amplitude than are corresponding signals which are received in the guard antenna 2. Correspondingly, sidelobe return signals received in the main antenna sidelobes are of a lower amplitude than the corresponding sidelobe return signals received by the guard antenna. Typically, the main channel 10 and the guard channel 20 contain filter banks 9 and 12 respectively. These filter banks are effective in acting as main beam clutter rejectors by filtering the output of the corresponding coherent receivers 7 and 8, and rejecting signals which are doppler shifted by an amount corresponding to the stationary clutter targets in the main beam with respect to the speed of the radar-carrying aircraft. Since the main beam clutter has a predictable doppler shift determined by the speed of the radar-carrying aircraft with respect to the stationary clutter targets in the main beam, only signals which are doppler shifted away from the main beam clutter frequency are passed through the filter banks in each channel.
Sidelobe clutter results from ground and object reflections of signals which are received in the sidelobes. Discrete clutter (also called point clutter) is a high amplitude return which occurs when a transmitted pulse reflects off a building or similar structure and is reflected back to the aircraft. When such a strong return occurs in the sidelobes (discrete sidelobe clutter) the doppler shift is different from the main beam returns. Therefore, since sidelobe clutter is shifted differently than main beam clutter, it will be passed by the filter banks in each channel along with any true target return signals. As discussed previously, due to the antenna characteristics, the guard channel 20 receives the sidelobe clutter with a higher amplitude than does the main channel 10. On that principle, the prior art embodiment shown in FIG. 1A, operates so that the detection of the sidelobe clutter in the guard channel 20 is used to cancel or blank out the correspondingly detected clutter which occurs in the sidelobes in the main channel 10. Since main beam clutter is removed by the filter bank 9, only the true target returns from the main beam, and the area and discrete clutter from the sidelobes and noise are passed by the filter bank 9. These signals are fed into a conventional constant false alarm rate (CFAR) threshold circuit 11. The CFAR threshold circuit has an adaptive threshold level which raises or lowers to block out area sidelobe clutter, but is ineffective against strong discrete sidelobe clutter signals.
The guard channel 20 normally receives the true target returns at an amplitude far reduced from that of the main beam reception, and the effect of the guard channel is to cause cancellation of only the discrete sidelobe clutter returns since these signals in the guard channel are higher than the sidelobe reception in the main channel 10. The output of the guard channel 20 is fed into the amplitude comparison and blanking circuit 13, wherein the amplitudes of the discrete doppler shifted signals corresponding in frequency are compared. Where the amplitude of the guard channel signal exceeds the corresponding signal amplitude in the main channel, that discrete frequency is blanked. Following the blanking operation, the range ambiguity resolver 15 receives the true target signals which are ambiguous in range, and resolves the range. If a plurality of different PRF's are transmitted, the resolver 15 divides the PRF's into a predetermined number of range cells and correlates the reception in corresponding range cells of each PRF. The signals which occupy corresponding range cells in each of the other PRF's are correlated, and the true target return signal is assigned to a range cell common to each PRF, thereby resolving the range ambiguity. The target is then displayed in a conventional manner on a PPI display, according to the resolved unambiguous range measurement, etc.
Problems with the prior art guard channel receiver system as described heretofore are obvious, since the guard channel involves a separate antenna and receiver system resulting in a duplication of hardware and instrumentation, which is not only expensive but adds additional weight to an aircraft environment.